Aryl hydrocarbon hydroxylase (AHH) metabolizes several important carcinogens. We have confirmed that the human population varies in the levels of this enzyme and in AHH inducibility. We confirmed that most of this variation is genetically determined. Induced AHH activity varies with the season of the year with summer values being 4-5 fold higher than winter values. Various experiments indicate that the seasonal variation is a variable in people rather than in the laboratory environment. Future experiments will attempt to map AHH in the human genome and to screen for genetic polymorphisms of other enzymes that metabolize carcinogens.